1. Field of the Invention
The present invention relates to a control circuit for an induction motor, more particularly, to a control circuit for an induction motor which estimates resistance of a rotor in the motor and controls three-phase current supplied to the motor to obtain a dynamic high performance characteristic of the motor.
For example, such a control circuit for an induction motor is well-known in Japanese patent laid-open publication No. Sho 57-180387. The control circuit proposed in the publication comprises a variable voltage/variable frequency controller (VVVF) arranged at the side of the power supply of an induction motor IM, a velocity operator SOP obtains a difference between a velocity signal NF designating the rotating velocity of the motor IM, detected by a velocity generator TG and a velocity command signal NR, a current operator IOP obtains current value of the motor IM on the basis of the induced current signal I2 from the velocity operator SOP and an exciting current IO and controls the output voltage of the variable voltage/variable frequency controller VVVF by using the current value, an adder ADD adds the velocity signal NF and an effective current I2 outputted from the velocity operator SOP and controls the output frequency of a voltage frequency converter VFC having its output supplied to a frequency controller FCT, a comparator CP feeds a primary current signal IF back to the velocity operator SOP in accordance with the output signal of a polarity detector PD when the primary current IF flowing through a rectifier REC, detected by a current transformer CT disposed at the output side of the variable voltage/variable frequency controller VVVF, is larger than the previously set value, a differentiator DF differentiates the output of the adder ADD and feeds the differentiated result back to the velocity operator SOP. An operator OP operates the primary current signal IF flowing through the rectifier REC and the output signal of the current operator IOP and supplies the operated result to a voltage controller VCT producing a voltage control signal to be supplied to the variable voltage/variable frequency controller VVVF, as shown in FIG. 1.
With the controller circuit for the induction motor thus constructed, the motor is controlled such that it is rotated with a predetermined slip in accordance with a frequency of a power supply voltage. Accordingly, when the velocity of the motor IM is changed for an acceleration or deceleration, it is difficult to maintain the slip constantly. Furthermore, during a change in the velocity, the slip varies and the rotating velocity of the motor IM is also unsuitable.
To solve the drawbacks, a vector control technique is considered.
According to the vector control technique, a slipping angular velocity must be operated when inputting a synchronous angular velocity. Therefore, if parameter a which is greatly variable depending on the variation of temperature, such as a rotating resistance of the motor IM, is employed for operating the slipping angular velocity, then the amount of slip is operated incorrectly. As a result, there is a problem in that efficiency of the motor IM cannot be normally assured.